1. Field of the Disclosure
The present invention relates generally to power supplies, and in particular but not exclusively, relates to controllers for switched mode power supplies.
2. Background
Switch mode power supplies are used in a wide variety of household or industrial appliances that require a regulated direct current (dc) voltage for their operation. There are a wide variety of known dc-dc converter topologies using PWM (pulse width modulation) or PFM (pulse frequency modulation) control to regulate output voltage.
One type of dc-dc converter topology is a resonant switched mode power converter. A resonant controller included in a resonant switched mode power converter controller with PFM utilizes resonant properties to provide natural soft switching in a closed loop to regulate the output. A resonant controller with PFM senses the power supply output and controls it by varying the switching frequency. An advantage of a resonant controller with PFM is that with the natural soft switching, there is reduced switching loss compared to non-resonant power converter topologies. Another advantage is that resonant controllers with PFM can be designed to operate with higher frequencies in a smaller package sizes.
Among a variety of resonant switched mode power converters are high frequency (HF) transformer isolated LLC converters, which have become increasingly popular in recent years. LLC resonant converters utilize the resonance between two inductors and a capacitor to provide natural soft switching. LLC resonant converters save on cost and size by utilizing the magnetizing and leakage inductance of the HF transformer as part of their resonance components. One disadvantage of some resonant converters is that the required wide range of frequency control result in more complicated electromagnetic interference (EMI) filter designs. However, with the gain characteristics of LLC resonant converters, output regulation can be achieved with a narrow band of frequency control.
To interface with low frequency (e.g. 60 or 50 Hz) alternating current (ac) networks, dc-dc LLC resonant converters require an ac-dc front stage. With continuing efforts to conserve energy, there are increasingly tighter obligatory standards for the overall efficiency, low load/no load and standby power consumption in power converters.
One known method for reducing power loss at low load and no load conditions in LLC resonant converters is by entering a mode of operation commonly known as “Burst Mode Control” by minimizing the switching activity and the power consumption during the low load and no load conditions. However, known LLC resonant converters having a burst mode control operation use the maximum operating frequency, which happens at lowest possible load, as the burst frequency, which results in a higher switching loss and result in the power converter not being in compliance with the tight values required by regulatory enforcement agencies for no load/standby power consumption.